Unsupervised, supervised and reinforced learning via spiking computation

ABSTRACT

The present invention relates to unsupervised, supervised and reinforced learning via spiking computation. The neural network comprises a plurality of neural modules. Each neural module comprises multiple digital neurons such that each neuron in a neural module has a corresponding neuron in another neural module. An interconnection network comprising a plurality of edges interconnects the plurality of neural modules. Each edge interconnects a first neural module to a second neural module, and each edge comprises a weighted synaptic connection between every neuron in the first neural module and a corresponding neuron in the second neural module.

GOVERNMENT RIGHTS

This invention was made with Government support under HR0011-09-C-0002awarded by Defense Advanced Research Projects Agency (DARPA). TheGovernment has certain rights in this invention.

BACKGROUND

The present invention relates to neuromorphic and synaptroniccomputation, and in particular, unsupervised, supervised and reinforcedlearning via spiking computation.

Neuromorphic and synaptronic computation, also referred to as artificialneural networks, are computational systems that permit electronicsystems to essentially function in a manner analogous to that ofbiological brains. Neuromorphic and synaptronic computation do notgenerally utilize the traditional digital model of manipulating 0s and1s. Instead, neuromorphic and synaptronic computation create connectionsbetween processing elements that are roughly functionally equivalent toneurons of a biological brain. Neuromorphic and synaptronic computationmay comprise various electronic circuits that are modeled on biologicalneurons.

In biological systems, the point of contact between an axon of a neuronand a dendrite on another neuron is called a synapse, and with respectto the synapse, the two neurons are respectively called pre-synaptic andpost-synaptic. The essence of our individual experiences is stored inconductance of the synapses. The synaptic conductance changes with timeas a function of the relative spike times of pre-synaptic andpost-synaptic neurons, as per spike-timing dependent plasticity (STDP).The STDP rule increases the conductance of a synapse if itspost-synaptic neuron fires after its pre-synaptic neuron fires, anddecreases the conductance of a synapse if the order of the two firingsis reversed.

BRIEF SUMMARY

The present invention relates to unsupervised, supervised and reinforcedlearning via spiking computation. In one embodiment, the presentinvention provides a neural network comprising a plurality of neuralmodules, wherein each neural module comprises multiple digital neuronssuch that each neuron in a neural module has a corresponding neuron inanother neural module. An interconnection network comprising a pluralityof edges interconnects the plurality of neural modules, wherein eachedge interconnects a first neural module to a second neural module, andeach edge comprises a weighted synaptic connection between every neuronin the first neural module and a corresponding neuron in the secondneural module.

In another embodiment, the present invention provides a methodcomprising producing spiking computation in a neural network comprisinga plurality of neural modules interconnected via weighted synapticconnections in an interconnection network, wherein each neural modulecomprises multiple digital neurons such that every neuron in a firstneural module is connected to a corresponding neuron in a second neuralmodule via a weighted synaptic connection. The spiking computationcomprises generating signals which define a set of time steps foroperation of the neuron. At each time step, each neuron based on itsoperational state determines whether to generate a firing event inresponse to firing events received as input signals from neurons inother neural modules. Each said input signal is weighted by the weightedsynaptic connection communicating said input signal to said neuron.

These and other features, aspects and advantages of the presentinvention will become understood with reference to the followingdescription, appended claims and accompanying figures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a neural network implementing an embodiment ofspiking computation in accordance with the present invention;

FIG. 2A illustrates the structure of a neural module, in accordance withan embodiment of the invention;

FIG. 2B illustrates the weighted synaptic connections interconnectingone neural module to another neural module, in accordance with anembodiment of the invention;

FIG. 3A illustrates an example input provided to a neuron in a neuralnetwork, in accordance with an embodiment of the invention;

FIG. 3B illustrates a receptive field of a neuron in a neural network,in accordance with an embodiment of the invention;

FIG. 3C illustrates a projective field of a neuron in a neural network,in accordance with an embodiment of the invention;

FIG. 3D illustrates an example input provided to a neural networkintersecting with a receptive field and a projective field of a neuronin the neural network, in accordance with an embodiment of theinvention;

FIG. 3E illustrates an example input provided to a neural networkintersecting with a receptive field and a projective field of a neuronin the neural network, in accordance with an embodiment of theinvention;

FIG. 4 illustrates a directed graph representing a neural networktrained for unsupervised learning by implementing an embodiment ofspiking computation in accordance with the present invention;

FIG. 5 shows a matrix representing the weighted synaptic connections ofa neural network trained for unsupervised learning by implementing anembodiment of spiking computation in accordance with the presentinvention;

FIG. 6 illustrates a directed graph representing a neural networktrained for unsupervised learning by implementing an embodiment ofspiking computation in accordance with the present invention;

FIG. 7A illustrates an example input and an example output provided to aneuron in a neural network, in accordance with an embodiment of theinvention;

FIG. 7B illustrates a receptive field of a neuron in a neural network,in accordance with an embodiment of the invention;

FIG. 7C illustrates a projective field of a neuron in a neural network,in accordance with an embodiment of the invention;

FIG. 7D illustrates an example input and an example output provided to aneural network intersecting with a receptive field and a projectivefield of a neuron in the neural network, in accordance with anembodiment of the invention;

FIG. 7E illustrates an example input and an example output provided to aneural network intersecting with a receptive field and a projectivefield of a neuron in the neural network, in accordance with anembodiment of the invention;

FIG. 8 illustrates a directed graph representing a feedforward learningstructure implementing an embodiment of spiking computation inaccordance with the present invention;

FIG. 9 illustrates a directed graph representing a feedback learningstructure implementing an embodiment of spiking computation inaccordance with the present invention;

FIG. 10 illustrates a directed graph representing a feedforwardunlearning structure implementing an embodiment of spiking computationin accordance with the present invention;

FIG. 11 illustrates a directed graph representing a feedback unlearningstructure implementing an embodiment of spiking computation inaccordance with the present invention;

FIG. 12 illustrates a directed graph representing a neural networkstructure trained for supervised, unsupervised and reinforcementlearning by implementing an embodiment of spiking computation inaccordance with the present invention;

FIG. 13 illustrates a directed graph representing a neural networkstructure trained for supervised, unsupervised and reinforcementlearning by implementing an embodiment of spiking computation inaccordance with the present invention;

FIG. 14 illustrates a directed graph representing a neural networkstructure trained for supervised, unsupervised and reinforcementlearning by implementing an embodiment of spiking computation inaccordance with the present invention;

FIG. 15 illustrates a directed graph representing a neural networkstructure trained for supervised, unsupervised and reinforcementlearning by implementing an embodiment of spiking computation inaccordance with the present invention;

FIG. 16 illustrates a directed graph representing a neural networkstructure trained for supervised, unsupervised and reinforcementlearning by implementing an embodiment of spiking computation inaccordance with the present invention;

FIG. 17 illustrates a directed graph representing a neural networkstructure trained in supervised, unsupervised and reinforcement learningby implementing an embodiment of spiking computation in accordance withthe present invention;

FIG. 18 illustrates a directed graph representing a neural networkstructure trained in supervised, unsupervised and reinforcement learningby implementing an embodiment of spiking computation in accordance withthe present invention;

FIG. 19 illustrates a directed graph representing a neural networkstructure trained in supervised, unsupervised and reinforcement learningby implementing an embodiment of spiking computation in accordance withthe present invention;

FIG. 20 illustrates a flowchart of an example process for training aneural network in supervised, unsupervised and reinforcement learning byimplementing an embodiment of spiking computation, in accordance with anembodiment of the invention;

FIG. 21 is a high level block diagram showing an information processingcircuit useful for implementing one embodiment of the present invention.

DETAILED DESCRIPTION

The present invention relates to unsupervised, supervised and reinforcedlearning via spiking computation. In one embodiment, the presentinvention provides a neural network comprising a plurality of neuralmodules, wherein each neural module comprises multiple digital neuronssuch that each neuron in a neural module has a corresponding neuron inanother neural module. An interconnection network comprising a pluralityof edges interconnects the plurality of neural modules, wherein eachedge interconnects a first neural module to a second neural module, andeach edge comprises a weighted synaptic connection between every neuronin the first neural module and a corresponding neuron in the secondneural module.

In another embodiment, the present invention provides a methodcomprising producing spiking computation in a neural network comprisinga plurality of neural modules interconnected via weighted synapticconnections in an interconnection network, wherein each neural modulecomprises multiple digital neurons such that every neuron in a firstneural module is connected to a corresponding neuron in a second neuralmodule via a weighted synaptic connection. The spiking computationcomprises generating signals which define a set of time steps foroperation of the neuron. At each time step, each neuron based on itsoperational state determines whether to generate a firing event inresponse to firing events received as input signals from neurons inother neural modules. Each said input signal is weighted by the weightedsynaptic connection communicating said input signal to said neuron.

The weights of all weighted synaptic connections in the interconnectionnetwork are adaptive as a function of the firing events of theinterconnected neurons. Each neuron has an operational state, and basedon its operational state, each neuron determines whether to generate afiring event in response to firing events received as input signals bythe neuron via weighted synaptic connections from neurons in otherneural modules. Each received input signal is weighted by the weightedsynaptic connection communicating the input signal to the neuron.

An interface module updates learning rules associated with each weightedsynaptic connection in the interconnection network. A learning rule forlearning false negatives or unlearning false positives is applied to aweighted synaptic connection in the interconnection network when aneuron interconnected via the weighted synaptic connection generates afiring event. The interface module updates the learning rules such thatthe neural network operates as one of more of: an auto-associativesystem, a hetero-associative system, and a reinforcement learningsystem.

Embodiments of the invention provide an adaptive neural network that caninterface in real-time with spatiotemporal sensorium and motorium tocarry out tasks of perception including unsupervised learning, actionincluding supervised learning, and cognition including reinforcementlearning, in a noise-robust, self-tuning, and self-configuring fashion.The present invention provides such a neural network by exploiting thesymmetry between feedforward and feedback processing pathways andbetween learning and unlearning so as to progressively transform falsepositives into false negatives. Embodiments of the invention furtherprovide a neural network that provides locality and massive parallelismto enable a low-power, compact hardware implementation.

The term digital neuron as used herein represents an architectureconfigured to simulate a biological neuron. A digital neuron createsconnections between processing elements that are roughly functionallyequivalent to neurons of a biological brain. As such, a neuromorphic andsynaptronic computation comprising digital neurons according toembodiments of the invention may include various electronic circuitsthat are modeled on biological neurons. Further, a neuromorphic andsynaptronic computation comprising digital neurons according toembodiments of the invention may include various processing elements(including computer simulations) that are modeled on biological neurons.Although certain illustrative embodiments of the invention are describedherein using digital neurons comprising electronic circuits, the presentinvention is not limited to electronic circuits. A neuromorphic andsynaptronic computation according to embodiments of the invention can beimplemented as a neuromorphic and synaptronic architecture comprisingcircuitry, and additionally as a computer simulation. Indeed,embodiments of the invention can take the form of an entirely hardwareembodiment, an entirely software embodiment or an embodiment containingboth hardware and software elements.

An external two-way communication environment may supply sensory inputsand consume motor outputs. Digital neurons implemented usingcomplementary metal-oxide-semiconductor (CMOS) logic gates receive spikeinputs and integrate them. The neurons include comparator circuits thatgenerate spikes when the integrated input exceeds a threshold. In oneembodiment, weighted synaptic connections are implemented usingtransposable 1-bit static random-access memory (SRAM) cells, whereineach neuron can be an excitatory or inhibitory neuron. Each learningrule on each neuron axon and dendrite are reconfigurable.

A neural network can be represented as a directed graph, with neuronsrepresented by vertices, and weighted synaptic connections betweenneurons represented by directed edges, each edge associated with asynaptic weight.

FIG. 1 illustrates a neural network 5 implementing an embodiment ofspiking computation according to the present invention, as describedherein. The example neural network 5 comprises multiple neural modules,such as neural modules 1, 2, and 3. Each neural module comprisesmultiple neurons (see, for example, neural module 200 in FIG. 2Acomprising neurons l_(i) ^(↑), l_(i) ^(↓), u_(i) ^(↑), and u_(i) ^(↓)).

The neural network 5 further comprises multiple edges, edges 6 and 7represented as junctions in FIG. 1. The edge 6 comprises a weightedsynaptic connection between every neuron in the neural module 1 and acorresponding neuron in the neural module 2. The edge 7 comprises aweighted synaptic connection between every neuron in the neural module 2and a corresponding neuron in the neural module 3.

Also shown in FIG. 1, the neural network 5 further comprises a controlmodule (“controller”) 9 that is connected to a clock 4. The clock 4produces clock signals used by the controller 9 to generate time-steps.The controller 9 divides each time-step into operational phases in theneural network for neuron updates, etc. In one embodiment, operation ofthe neural network 5 is driven by time-steps (e.g., based on biologicaltime-steps), wherein within a time-step multiple neuron updates andsynaptic connection updates are sequentially handled in a read phase anda write phase, respectively, utilizing the clock 4. Further, variabletime-steps may be utilized wherein the start of a next time-step may betriggered using handshaking signals whenever the neuron/synapseoperation of the previous time-step is completed. For externalcommunication, pipelining may be utilized wherein load inputs,neuron/synapse operation, and send outputs are pipelined (thiseffectively hides the input/output operating latency).

Also shown in FIG. 1, an input interface 172 provides input to someneurons in the neural modules. An output interface 173 receives outputfrom some neurons in the neural modules. An evaluation module 170evaluates the output of some neurons in the neural module. An interfacemodule 171 updates the learning rules of the synapses in the neuralnetwork circuit. The neural network 5 can comprise more than one outputinterface, one input interface, one evaluation module and/or oneinterface module.

FIG. 2A illustrates the structure of a neural module 200 (an example ofneural module 1 in FIG. 1), in accordance with an embodiment of theinvention. Each neural module comprises multiple neurons. For instance,the neural module 200 comprises four neurons, neurons l_(i) ^(↑), l_(i)^(↓), u_(i) ^(↑), and u_(i) ^(↓). There are four types of neurons: aneuron in a learning, bottom-up pathway; a neuron in a learning,top-down pathway; a neuron in an unlearning, bottom-up pathway; and, aneuron in an unlearning, top-down pathway.

In FIG. 2A, the neuron l_(i) ^(↑) is a neuron in a learning, bottom-uppathway (e.g. learning, bottom-up pathway 101 in FIG. 8), the neuronl_(i) ^(↓) is a neuron in a learning, top-down pathway (e.g. learning,top-down pathway 111 in FIG. 9), the neuron u_(i) ^(↑) is a neuron in anunlearning, bottom-up pathway (e.g. unlearning, bottom-up pathway 121 inFIG. 10), and the neuron u_(i) ^(↓) is a neuron in an unlearning,top-down pathway (e.g. unlearning, top-down pathway 131 in FIG. 11).

FIG. 2B illustrates a neural module 202 (an example of neural module 1in FIG. 1) interconnected to a neural module 203 (an example of neuralmodule 2 in FIG. 1), according to an embodiment of the invention. Eachneural module 202, 203 comprises exactly four neurons. The neural module202 comprises neurons l_(i) ^(↑), l_(i) ^(↓), u_(i) ^(↑), and u_(i)^(↓), and the neural module 203 comprises neurons l_(i+1) ^(↑), l_(i+1)^(↓), u_(i+1) ^(↑), and u_(i+1) ^(↓). Each neuron l_(i) ^(↑), l_(i)^(↓), u_(i) ^(↑), and u_(i) ^(↓) in the neural module 202 has aone-to-one relationship with neuron l_(i+1) ^(↑), l_(i+1) ^(↓), u_(i+1)^(↑), and u_(i+1) ^(↓) in the neural module 203, respectively.

As shown in FIG. 2B, an edge 201 (an example of edge 6 in FIG. 1)comprises weighted synaptic connections interconnecting neurons in theneural module 202 to corresponding neurons in the neural module 203. Aweighted synaptic connection 210 interconnects the neuron l_(i) ^(↑) tothe neuron l_(i+1) ^(↑), a weighted synaptic connection 220interconnects the neuron l_(i+1) ^(↓) to the neuron l_(i) ^(↓), aweighted synaptic connection 230 interconnects the neuron u_(i) ^(↑) tothe neuron u_(i+1) ^(↑), and a weighted synaptic connection 240interconnects the neuron u_(i+1) ^(↓) to the neuron u_(i) ^(↓). Allweighted synaptic connections between the neural modules 202 and 203have the same synaptic weight.

Each weighted synaptic connection is directed for information flowthrough the weighted synaptic connection. The edge 201 in FIG. 2Boperates in four contexts. In the first context, the neurons l_(i) ^(↑)and l_(i+1) ^(↑) are neurons in a learning, bottom-up pathway (e.g.learning, bottom-up pathway 101 in FIG. 8) and information flows fromthe neuron l_(i) ^(↑) to the neuron l_(i+1) ^(↑) through the weightedsynaptic connection 210. In the second context, the neurons l_(i) ^(↓)and l_(i+1) ^(↓) are neurons in a learning, top-down pathway (e.g.learning, top-down pathway 111 in FIG. 9) and information flows from theneuron l_(i+1) ^(↓) to the neuron l_(i) ^(↓) through the weightedsynaptic connection 220.

In the third context, the neurons u_(i) ^(↑) and u_(i+1) ^(↑) areneurons in an unlearning, bottom-up pathway (e.g. unlearning, bottom-uppathway 121 in FIG. 10) and information flows from the neuron u_(i) ^(↑)to the neuron u_(i+1) ^(↑) through the weighted synaptic connection 230.In the fourth context, the neurons u_(i) ^(↓) and u_(i+1) ^(↓) areneurons in an unlearning, top-down pathway (e.g. unlearning, top-downpathway 131 in FIG. 11) and information flows from the neuron u_(i+1)^(↓) to the neuron u_(i+1) ^(↓) through the weighted synaptic connection240.

With respect to the edge 201, the neurons l_(i) ^(↑), l_(i+1) ^(↓),u_(i) ^(↑), and u_(i+1) ^(↓) are pre-synaptic neurons and the neuronsl_(i+1) ^(↑), l_(i) ^(↓), u_(i+1) ^(↑), and u_(i) ^(↓) are post-synapticneurons. Arrow 251 represents information flowing from the neurons l_(i)^(↑) and u_(i) ^(↑) in the neural module 202 to the correspondingneurons l_(i+1) ^(↑) and u_(i+1) ^(↑) in the neural module 203,respectively. Arrow 252 represents information flowing from the neuronsl_(i+1) ^(↓) and u_(i+1) ^(↓) in the neural module 203 to thecorresponding neurons l_(i) ^(↓) and u_(i) ^(↓) in the neural module202, respectively. Arrow 253 represents information flowing to theneurons l_(i+1) ^(↓) and u_(i+1) ^(↓) in the neural module 203 fromcorresponding neurons in another neural module (not shown). Arrow 254represents information flowing from the neurons u_(i+1) ^(↑) and l_(i+1)^(↑) in the neural module 203 to corresponding neurons in another neuralmodule (not shown).

Unsupervised Learning

A neural network, in accordance with an embodiment of the invention, maybe trained for unsupervised learning (auto-associative memory). Theneural network learns a provided input and produces an approximation ofsaid input. For example, when a neural network including visual sensorsis provided a crescent moon shape as an input, the neural network learnsthe crescent moon shape by application of learning rules to the synapsesbased on spiking neurons, and reproduces the crescent moon shape as anapproximation of the input (approximate input).

FIG. 3A illustrates an example input provided to a neural network, inaccordance with an embodiment of the invention. As shown in input screen40, a crescent moon 45 is provided as input to some neurons in a neuralnetwork (see, for example, neurons in neuron population L₂ ^(↑) in FIG.4). FIG. 3B illustrates a receptive field of a neuron in a neuralnetwork, in accordance with an embodiment of the invention. A circle 46represents the region of the input screen 40 that a neuron receiving thecrescent moon 45 as input is receptive to (receptive field). The inputprocessed by the neural network will be a union of multiple suchreceptive fields.

FIG. 3C illustrates a projective field of a neuron in a neural network,in accordance with an embodiment of the invention. A rectangle 47represents the region in an approximate input screen 42 that a neuron ina neural network (see, for example, neurons in neuron population L₁ ^(↑)in FIG. 4) projects to (projective field). The approximate inputproduced by the neural network will be a union of multiple suchprojective fields.

FIG. 3D illustrates an example input provided to a neural networkintersecting with a receptive field and a projective field of a neuronin the neural network, in accordance with an embodiment of theinvention. The neurons receiving the crescent moon 45 as input arelistening to the region of the input screen 40 represented by the circle46 when it should only be listening to the region represented by thecrescent moon 45. Similarly, the neural network is producing therectangle 47 as its approximate input when it should be producing thecrescent moon 45.

FIG. 3E illustrates an example input provided to a neural networkintersecting with a receptive field and a projective field of a neuronin the neural network, in accordance with an embodiment of theinvention. In the input screen 40, a region 48 where the crescent moon45 and the circle 46 intersect represents an area of the crescent moon45 that the neural network has learned. A region 49 where the crescentmoon 45 does not intersect with the circle 46 (FIG. 3D) represents aregion of the input screen 40 that the neural network should belistening to but is not. The region 49 represents a false negative thatthe neural network should be trained to learn. A region 50 where thecircle 46 does not intersect with the crescent moon 45 represents aregion of the input screen 40 that the neural network should be ignoringbut is listening to instead. The region 50 represents a false positivethat the neural network should be trained to unlearn.

Also shown in FIG. 3E, in the approximate input screen 42, a region 51where the crescent moon 45 and the rectangle 47 (FIG. 3D) intersectrepresents an area of the crescent moon 45 that the neural network hasproduced. A region 52 where the crescent moon 45 does not intersect withthe rectangle 47 represents a region of the approximate input screen 42that the neural network should project but is not. The region 52represents a false negative that the neural network should be trained tolearn. An region 53 where the rectangle 47 does not intersect with thecrescent moon 45 represents a region of the approximate input screen 42that the neural network should be ignoring but is projecting instead.The region 53 is a false positive that the neural network should betrained to unlearn.

In one example implementation, to accomplish unsupervised learning in aneural network, the neural network may be trained to unlearn the falsepositives (see, for example, regions 50, 53 in FIG. 3E), learn the falsenegatives (see, for example, regions 49, 52 in FIG. 3E), and maintainwhat is correct (see, for example, regions 48, 51 in FIG. 3E). Using theneural network in FIG. 1, the present invention provides a neuralnetwork capable of self-tuning to achieve unsupervised learning bylearning the false negatives, unlearning the false positives, andmaintaining what is correct.

FIG. 4 illustrates a directed graph representing a neural network 60trained for unsupervised learning by implementing an embodiment ofspiking computation in accordance with the present invention. The neuralnetwork 60 comprises edges 64A, 64B, and 64C, pathways 61, 62, and 63,and neuron populations, U₁ ^(↑), U₂ ^(↑), L₁ ^(↑), L₂ ^(↑), L₁ ^(↓), andL₂ ^(↓). Each neuron population U₁ ^(↑), U₂ ^(↑), L₁ ^(↑), L₂ ^(↑), L₁^(↓), and L₂ ^(↓) comprises a set of neurons.

The pathway 61 is an unlearning, bottom-up pathway comprising the neuronpopulations U₁ ^(↑) and U₂ ^(↑). The pathway 61 is used in the neuralnetwork 60 to unlearn false positives (e.g. network-createdinformation). Let U_(i) ^(↑)={u_(i,1) ^(↑), u_(i,2) ^(↑), . . . ,u_(i,Ni) ^(↑)}, i=1, 2, denote the set of all neurons within a neuronpopulation in the unlearning, bottom-up pathway 61. The edge 64Acomprises a weighted synaptic connection between every neuron in theneuron population U₁ ^(↑) and a corresponding neuron in the neuronpopulation U₂ ^(↑).

The pathway 62 is a learning, bottom-up pathway comprising the neuronpopulations L₁ ^(↑) and L₂ ^(↑). The pathway 62 is used in the neuralnetwork 60 to learn false negatives (e.g. input). Let L_(i)^(↑)={l_(i,1) ^(↑), l_(i,2) ^(↑), . . . , l_(i,Ni) ^(↑)}, i=1, 2, denotethe set of all neurons within a neuron population in the learning,bottom-up pathway 62. The edge 64B comprises a weighted synapticconnection between every neuron in the neuron population L₁ ^(↑) and acorresponding neuron in the neuron population L₂ ^(↑). Input is providedto the neurons in the L₂ ^(↑) population. Noise is present in the inputto sculpt the receptive fields of the neurons in the neuron populationL₁ ^(↑).

The pathway 63 is a learning, top-down pathway comprising the neuronpopulations L₁ ^(↓) and L₂ ^(↓). The pathway 63 is used in the neuralnetwork 60 to learn false negatives (e.g. input). Let L_(i)^(↓)={l_(i,1) ^(↓), l_(i,2) ^(↓), . . . , l_(i,Ni) ^(↓)}, i=1, 2, denotethe set of all neurons within a neuron population in the learning,top-down pathway 63. The edge 64C comprises a weighted synapticconnection between every neuron in the neuron population L₁ ^(↓) and acorresponding neuron in the neuron population L₂ ^(↓).

The synaptic weight of a weighted synaptic connection between a neuronl_(2,α) ^(↑) and a neuron l_(1,β) ^(↓) is a weight w_(α,β). The synapticweight of a weighted synaptic connection between a neuron l_(1,β) ^(↓)and a neuron l_(2,α) ^(↓) is the weight w_(α,β). The synaptic weight ofa weighted synaptic connection between a neuron u_(2,α) ^(↑) and aneuron u_(1,β) ^(↑) is the weight w_(α,β). The same weight w_(α,β) isused in three different contexts. In FIG. 4, the synaptic weight ofevery weighted synaptic connection in the edge 64A is represented by amatrix W. The matrix W is also used to represent the synaptic weight ofevery weighted synaptic connection in the edge 64B. The synaptic weightof every weighted synaptic connection in the edge 64C is represented bythe transpose of the matrix W, W^(T). Thus, the matrix W is used inthree different contexts.

In one time step, each neuron in every neuron module updates itsoperational state and, based on its operational state, each said neurondetermines whether to generate a firing event in response to firingevents received as input signals by said neuron via weighted synapticconnections from neurons in other neural modules. Each said input signalis weighted by the weight of the weighted synaptic connectioncommunicating said input signal to said neuron. The synaptic weightassociated with each weighted synaptic connection is adapted as afunction of the firing events of the interconnected neurons.

For instance, in the neural network 60, each time step is divided intothree different phases. The operational state of each neuron in theneuron populations L₂ ^(↑) and L₁ ^(↑) is updated in one phase of a timestep. The operational state of each neuron in the neuron populations L₂^(↓) and L₁ ^(↓) is updated in another phase of the same time step. Theoperational state of each neuron in the neuron populations U₂ ^(↑) andU₁ ^(↑) is updated in yet another phase of the same time step.

The neural network 60 propagates information in a loop. The firingevents generated by the neurons in the L₁ ^(↑) population are copied andprovided as input to the neurons in the L₁ ^(↓) population. The firingevents generated by the neurons in the L₂ ^(↑) population are alsocopied and provided as input to the neurons in the U₂ ^(↓) population.

FIG. 5 shows a matrix 70 that represents the weighted synapticconnections of the neural network 60 in FIG. 4, in accordance with anembodiment of the invention. Each row and each column of the matrix 70represents the pre-synaptic and post-synaptic populations of the neuralnetwork 60, respectively. Row 1, column 0 of the matrix 70 indicatesthat the synaptic weights of the weighted synaptic connections betweenthe neuron populations U₂ ^(↑) and U₁ ^(↑) is represented by the matrixW. Likewise, row 5, column 4 of the matrix 70 indicates that thesynaptic weights of the weighted synaptic connections between the neuronpopulations L₂ ^(↓) and L₁ ^(↓) is also represented by the matrix W. Row2, column 3 of the matrix 70 indicates that the synaptic weights of theweighted synaptic connections between the neuron populations L₁ ^(↓) andL₂ ^(↓) is represented by the transpose of matrix W, W^(T). The I* inrow 3, column 1 of the matrix 70 represents the copying of the firingevents produced by the neurons in the neuron population L₂ ^(↓) toprovide as input to the U₂ ^(↑) population. Similarly, the I* in row 4,column 2 of the matrix 70 represents the copying of the firing eventsproduced by the neurons in the neuron population L₁ ^(↑) to provide asinput to the L₁ ^(↓) population.

FIG. 6 illustrates a directed graph representing the neural network 60in FIG. 5, in accordance with an embodiment of the invention. Learningrules are applied to the weighted synaptic connections in the same timestep that each neuron in each neural module updates its operationalstates. The learning rules are applied based on the firing activity ofinterconnected neurons in a previous time step.

As shown in FIG. 6, heavy learning occurs by applying a STDP learningrule to the weighted synaptic connections in the edge 64B. The firingevents of the neurons in the neuron population L₂ ^(↑) is amplified asit propagates to the neurons in the neuron population L₁ ^(↑) via theweighted synaptic connections in the edge 64B. In this manner, the inputprovided to the neurons in the neuron population L₂ ^(↑) is learned bythe neural network 60.

The firing events produced by the neurons in the neuron population L₂^(↓), however, are network-created, and this network-created informationmay be unlearned by the neural network 60. As such, the firing eventsproduced by the L₂ ^(↓) population are copied and provided as input tothe neurons in the neuron population U₂ ^(↑). An anti-STDP learning ruleis applied to the weighted synaptic connections in the edge 64A tounlearn the network-created information. The learning rules STDP andanti-STDP are augmented by Hebbian (Hebb) and anti-Hebbian (anti-Hebb),respectively.

After training, the neural network 60 can reproduce the input providedto it as an approximate input. For instance, if the neural network 60 isprovided with the crescent moon 45 (FIG. 3A) as an input, the neuralnetwork 60 can reproduce the crescent moon 45 as an approximate input.

Supervised Learning

A neural network, in accordance with an embodiment of the invention, maybe trained for supervised learning (hetero-associative memory). Theneural network learns to associate a provided input with a providedoutput (the output is essentially another provided input to the neuralnetwork). When fed the provided input, the neural network produces anapproximation of the output. When fed the provided output, the neuralnetwork produces an approximation of the input. For example, when aneural network including visual sensors is provided a crescent moonshape as a provided input and a cross shape as a provided output, theneural network learns the crescent moon shape and the cross shape byapplication of learning rules to the synapses based on spiking neurons,and reproduces the crescent moon shape as an approximation of the input(approximate input) and the cross shape as an approximation of theoutput (approximate output).

FIG. 7A illustrates an example input and an example output provided to aneural network, in accordance with an embodiment of the invention. Aninput screen 70 provides a crescent moon 75 as input to the neuralnetwork. An output screen 71 provides a cross 76 as input to the neuralnetwork.

FIG. 7B illustrates a receptive field of a neuron in a neural network,in accordance with an embodiment of the invention. A circle 77 in theinput screen 70 represents the region of the input screen 70 that aneuron in the neural network is receptive to (receptive field). Theinput processed by the neural network will be a union of multiple suchreceptive fields. A diamond 78 in the output screen 71 represents theregion of the output screen 71 that a neuron in the neural network isreceptive to (receptive field). The output processed by the neuralnetwork will be a union of multiple such receptive fields.

FIG. 7C illustrates a projective field of a neuron in a neural network,in accordance with an embodiment of the invention. A rectangle 86 in anapproximate input screen 72 represents the projective field of a neuronin the neural network. The approximate input produced by the neuralnetwork will be a union of multiple such projective fields. A triangle90 in an approximate output screen 73 represents the projective field ofa neuron in the neural network. The approximate output produced by theneural network will be a union of multiple such projective fields.

FIG. 7D illustrates an example input and an example output provided to aneural network intersecting with a receptive field and a projectivefield of a neuron in the neural network, in accordance with anembodiment of the invention. In input screen 70, the neural network islistening to the region represented by the circle 77 when it should onlybe listening to region represented by the crescent moon 75. Similarly,in output screen 71, the neural network is listening to the regionrepresented by the diamond 78 (FIG. 7C) when it should only be listeningto the region represented by the cross 76.

Also shown in FIG. 7D, in approximate input screen 72, the neuralnetwork produces the rectangle 86 as its approximate input when itshould be producing the crescent moon 75. Similarly, in approximateoutput screen 73, the neural network produces the triangle 90 as itsapproximate output when it should be producing the cross 76.

FIG. 7E illustrates an example input and an example output provided to aneural network intersecting with a receptive field and a projectivefield of a neuron in the neural network, in accordance with anembodiment of the invention. In the input screen 70, a region 79 wherethe crescent moon 75 and the circle 77 (FIG. 7C) intersect represents anarea of the crescent moon 75 that the neural network has learned. Aregion 80 where the crescent moon 75 does not intersect with the circle77 represents a region of the input screen 70 that the neural networkshould be listening to but is not. The region 80 is a false negativethat the neural network should be trained to learn. A region 81 wherethe circle 77 does not intersect with the crescent moon 75 represents aregion of the input screen 70 that the neural network should be ignoringbut is listening to instead. The region 81 is a false positive that theneural network should be trained to unlearn.

Also shown in FIG. 7E, in the output screen 71, a region 82 where thecross 76 and the diamond 78 intersect represents an area of the cross 76that the neural network has learned. A region 83 where the cross 76 doesnot intersect with the diamond 78 represents a region of the outputscreen 71 that the neural network should be listening to but is not. Theregion 83 is a false negative that the neural network should be trainedto learn. A region 85 where the diamond 78 does not intersect with thecross 76 represents a region of the output screen 71 that the neuralnetwork should be ignoring but is listening to instead. The region 85 isa false positive that the neural network should be trained to unlearn.

Further shown in FIG. 7E, in the approximate input screen 72, a region87 where the crescent moon 75 and the rectangle 86 intersect representsan area of the crescent moon 75 that the neural network has produced. Aregion 88 where the crescent moon 75 does not intersect with therectangle 86 represents a region of the approximate input screen 72 thatthe neural network should project but is not. The region 88 is a falsenegative that the neural network should be trained to learn. A region 89where the rectangle 86 intersects with the crescent moon 75 represents aregion of the approximate input screen 72 that the neural network shouldbe ignoring but is projecting instead. The region 89 is a false positivethat the neural network should be trained to unlearn.

Also shown in FIG. 7E, in the approximate output screen 73, a region 91where the cross 76 and the triangle 90 intersect represents an area ofthe cross 76 that the neural network has produced. A region 92 where thecross 76 does not intersect with the triangle 90 represents a region ofthe approximate output screen 73 that the neural network should projectbut is not. The region 92 is a false negative that the neural networkshould be trained to learn. A region 93 where the triangle 90 does notintersect with the cross 76 represents a region of the approximateoutput screen that the neural network should be ignoring but isprojecting instead. The area 93 is a false positive that the neuralnetwork should be trained to unlearn.

In one example implementation, to accomplish supervised learning in aneural network, the neural network may be trained to unlearn the falsepositives (see, for example, regions 81, 85, 89, 93 in FIG. 7E), learnthe false negatives (see, for example, regions 80, 83, 88, 92 in FIG.7E), and maintain what is correct (see, for example, regions 79, 82, 87,91 in FIG. 7E). Using the neural network in FIG. 1, the presentinvention provides a neural network capable of self-tuning itself toachieve supervised learning by learning the false negatives, unlearningthe false positives, and maintaining what is correct.

FIG. 8 illustrates a directed graph representing a feedforward learningstructure 100 implementing an embodiment of spiking computation inaccordance with the present invention. The feedforward learningstructure 100 comprises a learning, bottom-up pathway 101 comprisingfour neuron populations, L₁ ^(↑), L₂ ^(↑), L₃ ^(↑), and L₄ ^(↑). Thepathway 101 further comprises three edges, 141B, 142B, and 143B.

Each neuron population in FIG. 8 comprises a set of neurons. Let L_(i)^(↑)={l_(i,1) ^(↑), l_(i,2) ^(↑), . . . , l_(i,Ni) ^(↑)}, i=1, 2, 3, 4,denote the set of all neurons within a neuron population in thelearning, bottom-up pathway 101. The synaptic weight from a neuronn_(i+1,α) ^(↑) to a neuron n_(i,β) ^(↑) is a weight w_(α,β) ^(i+1).Input is provided to the neurons in the neuron population L₄ ^(↑) and anapproximate output is produced by the neurons in the neuron populationL₁ ^(↑).

The edge 141B comprises a weighted synaptic connection between everyneuron in the neuron population L₄ ^(↑) and a corresponding neuron inthe neuron population L₃ ^(↑). The edge 142B comprises a weightedsynaptic connection between every neuron in the neuron population L₃^(↑) and a corresponding neuron in the neuron population L₂ ^(↑). Theedge 143B comprises a weighted synaptic connection between every neuronin the neuron population L₂ ^(↑) and a corresponding neuron in theneuron population L₁ ^(↑).

FIG. 9 illustrates a directed graph representing a feedback learningstructure 110 implementing an embodiment of spiking computation inaccordance with the present invention. The feedback learning structure110 comprises a learning, top-down pathway 111 comprising four neuronpopulations, L₁ ^(↓), L₂ ^(↓), L₃ ^(↓), and L₄ ^(↓). The pathway 111further comprises three edges, 141C, 142C, and 143C.

Each neuron population in FIG. 9 comprises a set of neurons. Let L_(i)^(↓)={l_(i,1) ^(↓), l_(i,2) ^(↓), . . . , l_(i,Ni) ^(↓)}, i=1, 2, 3, 4,denote the set of all neurons within a neuron population in thelearning, top-down pathway 110. The synaptic weight from a neuronn_(i,β) ^(↓) to a neuron n_(i+1,α) ^(↓) is a weight w_(α,β) ^(i+1).Output is provided to the neurons in the neuron population L₁ ^(↓) andan approximate input is produced by the neurons in the neuron populationL₄ ^(↓).

The edge 141C comprises a weighted synaptic connection between everyneuron in the neuron population L₄ ^(↓) and a corresponding neuron inthe neuron population L₃ ^(↓). The edge 142C comprises a weightedsynaptic connection between every neuron in the neuron population L₃^(↓) and a corresponding neuron in the neuron population L₂ ^(↓). Theedge 143C comprises a weighted synaptic connection between every neuronin the neuron population L₂ ^(↓) and a corresponding neuron in theneuron population L₁ ^(↓).

FIG. 10 illustrates a directed graph representing a feedforwardunlearning structure 120 implementing an embodiment of spikingcomputation in accordance with the present invention. The feedforwardunlearning structure 120 comprises an unlearning, bottom-up pathway 121comprising four neuron populations, U₁ ^(↑), U₂ ^(↑), U₃ ^(↑), and U₄^(↑). The pathway 121 further comprises three edges, 141A, 142A, and143A.

Each neuron population in FIG. 10 comprises a set of neurons. Let U_(i)^(↑)={u_(i,1) ^(↑), u_(i,2) ^(↑), . . . , u_(i,Ni) ^(↑)}, i=1, 2, 3, 4,denote the set of all neurons within a neuron population in theunlearning, bottom-up pathway 121. The synaptic weight from a neuronu_(i+1,α) ^(↑) to a neuron u_(i,β) ^(↑) is a weight w_(α,β) ^(i+1).

The edge 141A comprises a weighted synaptic connection between everyneuron in the neuron population U₄ ^(↑) and a corresponding neuron inthe neuron population U₃ ^(↑). The edge 142A comprises a weightedsynaptic connection between every neuron in the neuron population U₃^(↑) and a corresponding neuron in the neuron population U₂ ^(↑). Theedge 143A comprises a weighted synaptic connection between every neuronin the neuron population U₂ ^(↑) and a corresponding neuron in theneuron population U₁ ^(↑).

FIG. 11 illustrates a directed graph representing a feedback unlearningstructure 130 by implementing an embodiment of spiking computation inaccordance with the present invention. The feedback unlearning structure130 comprises an unlearning, top-down pathway 131 comprising four neuronpopulations, U₁ ^(↓), U₂ ^(↓), U₃ ^(↓), and U₄ ^(↓). The pathway 131further comprises three edges, 141D, 142D, and 143D.

Each neuron population in FIG. 11 comprises a set of neurons. Let U_(i)^(↓)={u_(i,1) ^(↓), u_(i,2) ^(↓), . . . , u_(i,Ni) ^(↓)}, i=1, 2, 3, 4,denote the set of all neurons within a neuron population in theunlearning, top-down pathway 131. The synaptic weight from a neuronu_(i,β) ^(↓) to a neuron u_(i+1,α) ^(↓) is a weight w_(α,β) ^(i+1).

The edge 141D comprises a weighted synaptic connection between everyneuron in the neuron population U₄ ^(↓) and a corresponding neuron inthe neuron population U₃ ^(↓). The edge 142D comprises a weightedsynaptic connection between every neuron in the neuron population U₃^(↓) and a corresponding neuron in the neuron population U₂ ^(↓). Theedge 143D comprises a weighted synaptic connection between every neuronin the neuron population U₂ ^(↓) and a corresponding neuron in theneuron population U₁ ^(↓).

FIG. 12 illustrates a neural network structure 140 trained forsupervised, unsupervised and reinforcement learning by implementing anembodiment of spiking computation in accordance with the presentinvention. The neural network structure 140 comprises edges 141A, 141B,141C, 141D, 142A, 142B, 142C, 142D, 143A, 143B, 143C, and 143D, alearning, bottom-up pathway 101, a learning, top-down pathway 111, anunlearning, bottom-up pathway 121, an unlearning, top-down pathway 131,and sixteen neuron populations, L₁ ^(↑), L₂ ^(↑), L₃ ^(↑), L₄ ^(↑), L₁^(↓), L₂ ^(↓), L₃ ^(↓), L₄ ^(↓), U₁ ^(↑), U₂ ^(↑), U₃ ^(↑), U₄ ^(↑), U₁^(↓), U₂ ^(↓), U₃ ^(↓), and U₄ ^(↓).

As in FIG. 8, the learning, bottom-up learning pathway 101 in FIG. 12comprises the neuron populations L₁ ^(↑), L₂ ^(↑), L₃ ^(↑), and L₄ ^(↑),and the edges 141B, 142B, and 143B. An input interface 172 (FIG. 1)provides input to the neurons in the neuron population L₄ ^(↑). Theneurons in the neuron population L_(i) ^(↑) produce an approximateoutput. Firing/output events of the neurons in the neuron populationL_(i) ^(↑) are provided to the output interface 173 (FIG. 1). Thepathway 101 can be used in the neural network 140 to learn falsenegatives (e.g. input).

For example, the neuron population L₄ ^(↑) population can include theneuron l_(i) ^(↑) of the neural module 202 in FIG. 2B. The neuronpopulation L₃ ^(↑) population can include the neuron l_(i+1) ^(↑) of theneural module 203 in FIG. 2B. The edge 141B can include the weightedsynaptic connection 210 interconnecting the neuron l_(i) ^(↑) to theneuron l_(i+1) ^(↑) in FIG. 2B.

As in FIG. 9, the learning, top-down pathway 111 in FIG. 12 comprisesthe neuron populations L₁ ^(↓), L₂ ^(↓), L₃ ^(↓), and L₄ ^(↓), and theedges 141C, 142C, and 143C. Output is provided to the neurons in theneuron population L₁ ^(↓), and the neurons in the neuron population L₄^(↓) produce an approximate input. The pathway 111 can be used in theneural network 140 to learn false negatives (e.g. input).

For example, the neuron population L₄ ^(↓) population can include theneuron l_(i) ^(↓) of the neural module 202 in FIG. 2B. The neuronpopulation L₃ ^(↓) population can include the neuron l_(i+1) ^(↓) of theneural module 203 in FIG. 2B. The edge 141C can include the weightedsynaptic connection 220 interconnecting the neuron l_(i) ^(↓) to theneuron l_(i+1) ^(↓) in FIG. 2B.

As in FIG. 10, the unlearning, bottom-up pathway 121 in FIG. 12comprises the neuron populations U₁ ^(↑), U₂ ^(↑), U₃ ^(↑), and U₄ ^(↑),and the edges 141A, 142A, and 143A. The pathway 121 can be used in theneural network 140 to unlearn false positives (e.g. network-createdinformation).

For example, the neuron population U₄ ^(↑) population can include theneuron of the neural module 202 in FIG. 2B. The neuron population U₃^(↑) population can include the neuron u_(i+1) ^(↑) of the neural module203 in FIG. 2B. The edge 141A can include the weighted synapticconnection 230 interconnecting the neuron u_(i) ^(↑) to the neuronu_(i+1) ^(↑) in FIG. 2B.

As in FIG. 11, the unlearning, top-down pathway 131 in FIG. 12 comprisesthe neuron populations U₁ ^(↓), U₂ ^(↓), U₃ ^(↓), and U₄ ^(↓), and theedges 141D, 142D, and 143D. The pathway 131 can be used in the neuralnetwork 140 to unlearn false positives (network-created information).

For example, the neuron population U₄ ^(↓) population can include theneuron u_(i) ^(↓) of the neural module 202 in FIG. 2B. The neuronpopulation U₃ ^(↓) population can include the neuron u_(i+1) ^(↓) of theneural module 203 in FIG. 2B. The edge 141D can include the weightedsynaptic connection 240 interconnecting the neuron u_(i) ^(↓) to theneuron u_(i+1) ^(↓) in FIG. 2B.

In FIG. 12, the synaptic weight of every weighted synaptic connection inthe edge 141A is represented by a matrix W₁. The matrix W₁ is also usedto represent the synaptic weight of every weighted synaptic connectionin the edge 141B. The synaptic weight of every weighted synapticconnection in the edge 141C is represented by the transpose of thematrix W₁, W₁ ^(T). The transpose of the matrix W₁, W₁ ^(T), is alsoused to represent the synaptic weight of every weighted synapticconnection in the edge 141D. Thus, the matrix W₁ is used in fourdifferent contexts.

The synaptic weight of every weighted synaptic connection in the edge142A is represented by a matrix W₂. The matrix W₂ is also used torepresent the synaptic weight of every weighted synaptic connection inthe edge 142B. The synaptic weight of every weighted synaptic connectionin the edge 142C is represented by the transpose of the matrix W₂, W₂^(T). The transpose of the matrix W₂, W₂, is also used to represent thesynaptic weight of every weighted synaptic connection in the edge 142D.Thus, the matrix W₂ is used in four different contexts.

The synaptic weight of every weighted synaptic connection in the edge143A is represented by a matrix W₃. The matrix W₃ is also used torepresent the synaptic weight of every weighted synaptic connection inthe edge 143B. The synaptic weight of every weighted synaptic connectionin the edge 143C is represented by the transpose of the matrix W₃, W₃^(T). The transpose of the matrix W₃, W₃ ^(T), is also used to representthe synaptic weight of every weighted synaptic connection in the edge143D. Thus, the matrix W₃ is used in four different contexts.

Each matrix W₁, W₂, and W₃, and its transpose W₁ ^(T), W₂ ^(T), and W₃^(T), is doubly stochastic. The weights of all weighted synapticconnections in the neural network 140 are adaptive as a function of thefiring events of the interconnected neurons.

In one time step, each neuron in every neuron module updates itsoperational state and, based on its operational state, each said neurondetermines whether to generate a firing event in response to firingevents received as input signals by said neuron via weighted synapticconnections from neurons in other neural modules. Each said input signalis weighted by the weight of the weighted synaptic connectioncommunicating said input signal to said neuron.

For instance, in the neural network 140, each time step is divided intofour different phases. The operational state of each neuron in theneuron populations L₁ ^(↑), L₂ ^(↑), L₃ ^(↑), and L₄ ^(↑) is updated inone phase of a time step. The operational state of each neuron in theneuron populations L₁ ^(↓), L₂ ^(↓), L₃ ^(↓), and L₄ ^(↓) is updated ina second phase of the same time step. The operational state of eachneuron in the neuron populations U₁ ^(↓), U₂ ^(↓), U₃ ^(↓), and U₄ ^(↓)is updated in a third phase of the same time step. The operational stateof each neuron in the neuron populations U₁ ^(↑), U₂ ^(↑), U₃ ^(↑), andU₄ ^(↑) is updated in a fourth phase of the same time step.

The neural network 140 propagates information in a circular loop. Thefiring events generated by the neurons in the L₁ ^(↑) population arecopied and provided as input to the neurons in the L₁ ^(↑) population.The firing events generated by the neurons in the L₄ ^(↓) population arealso copied and provided as input to the neurons in the U₄ ^(↑)population.

FIG. 13 illustrates a neural network structure 140 trained forsupervised, unsupervised and reinforcement learning by implementing anembodiment of spiking computation in accordance with the presentinvention. Features of the input provided to the L₄ ^(↑) population,such as texture, color, etc., are extracted when the input propagatesfrom the L₄ ^(↑) population to the L₃ ^(↑) population via the learning,bottom-up pathway 101. Similarly, features of the output provided to theL₁ ^(↓) population are extracted when the output propagates from the L₁^(↓) population to the L₂ ^(↓) population via the learning, top-downpathway 111. Thus, the neural network structure 140 is trained toassociate the features of the input provided with the features of theoutput provided, and vice versa.

FIG. 14 illustrates a neural network structure 140 trained forsupervised, unsupervised and reinforcement learning by implementing anembodiment of spiking computation in accordance with the presentinvention. Learning rules are applied to the weighted synapticconnections in the same time step that each neuron in each neural moduleupdates its operational states. The learning rules are applied based onthe firing activity of interconnected neurons in a previous time step.The neural network 140 further comprises an interface module 171(FIG. 1) configured for updating learning rules associated with eachweighted synaptic connection in the interconnection network.

As shown in FIG. 14, heavy learning occurs by applying a STDP learningrule to the weighted synaptic connections in the edge 141B. The firingactivity of the neurons in the neuron population L₄ ^(↑) is amplified asit propagates to the neurons in the neuron population L₃ ^(↑) via theweighted synaptic connections in the edge 141B. In this manner, theinput provided to the neurons in the neuron population L₄ ^(↑) islearned by the neural network 140.

Similarly, as shown in FIG. 14, heavy learning occurs by applying a STDPlearning rule to the weighted synaptic connections in the edge 143C. Thefiring activity of the neurons in the neuron population L₁ ^(↓) isamplified as it propagates to the neurons in the neuron population L₂^(↓) via the weighted synaptic connections in the edge 143C. In thismanner, the output provided to the neurons in the neuron population L₁^(↓) is learned by the neural network 140.

The firing events produced by the neurons in the neuron population L₄^(↓), however, are network-created, and this network-created informationmay be unlearned by the neural network 140. As such, the firing eventsproduced by the L₄ ^(↓) population are copied and provided as input tothe neurons in the neuron population U₂ ^(↑). An anti-STDP learning ruleis applied to the weighted synaptic connections in the edge 141A tounlearn the network-created information.

Similarly, the firing events produced by the neurons in the neuronpopulation L₁ ^(↑), however, are network-created, and thisnetwork-created information may be unlearned by the neural network 140.As such, the firing events produced by the L₁ ^(↑) population are copiedand provided as input to the neurons in the neuron population U₁ ^(↓).An anti-STDP learning rule is applied to the weighted synapticconnections in the edge 143D to unlearn the network-created information.

The learning rules STDP and anti-STDP are augmented by Hebb andanti-Hebb, respectively. For instance, Hebb learning based on the firingactivity of the neurons in the neuron populations L₃ ^(↑) and L₂ ^(↓) isapplied to adjust the synaptic weights of the weighted synapticconnections in the neural network 140.

After training, the neural network 140 can produce the output as anapproximate output when provided with the input. Likewise, the neuralnetwork 140 can produce the input as an approximate input when providedwith the output. For instance, if the neural network 140 is providedwith the crescent moon 75 (FIG. 7A) as an input, the neural network 140can produce the cross 76 (FIG. 7A) as an approximate output.

FIG. 15 illustrates a neural network structure 140 trained forsupervised, unsupervised and reinforcement learning with learning rulesapplied, in accordance with an embodiment of the invention. Anti-Hebblearning based on the firing activity of the neurons in the neuronpopulations L₃ ^(↑) and U₂ ^(↓) is applied to adjust the synapticweights of the weighted synaptic connections in the neural network 140.

FIG. 16 illustrates a neural network structure 140 trained forsupervised, unsupervised and reinforcement learning with learning rulesapplied, in accordance with an embodiment of the invention. Anti-Hebblearning based on the firing activity of the neurons in the neuronpopulations U₃ ^(↑) and L₂ ^(↓) is applied to adjust the synapticweights of the weighted synaptic connections in the neural network 140.

Unsupervised learning can be realized as a sub-function of the neuralnetwork 140 in FIG. 12. To train the neural network 140 in unsupervisedlearning, the output provided to the L₁ ^(↓) population is set to theinput provided to the L₄ ^(↑) population.

Reinforcement Learning

FIG. 17 illustrates a neural network structure 140 trained insupervised, unsupervised and reinforcement learning by implementing anembodiment of spiking computation in accordance with the presentinvention. Reinforcement learning can be realized as a sub-function ofthe neural network 140 in FIG. 12. The neural network 140 furthercomprises an evaluation module 170 (FIG. 1) configured for evaluatingthe firing events (approximate output) generated by the neurons in theneuron population L₁ ^(↑).

If the approximate output is good, the evaluation module 170 copies theapproximate output and provides it as input to the neurons in the neuronpopulation L₁ ^(↓). A switch 160 in the neural network 140 closes topropagate the approximate output to the neurons in the neuron populationL₁ ^(↓). The approximate output is learned as it propagates down to theL₄ ^(↓) population via the learning, top-down learning pathway 111. Hebblearning based on the firing activity of the neurons in the neuronpopulations L₃ ^(↑) and L₂ ^(↓) is applied to adjust the synapticweights of the weighted synaptic connections in the neural network 140.

FIG. 18 illustrates a neural network structure 140 trained insupervised, unsupervised and reinforcement learning with learning rulesapplied, in accordance with an embodiment of the invention. If theapproximate output is bad, the evaluation module 170 copies theapproximate output and provides it as input to the neurons in the neuronpopulation U₁ ^(↓) population. A switch 161 in the neural network 140closes to propagate the approximate output to the neurons in the neuronpopulation U₁ ^(↓). The approximate output is unlearned as it propagatesdown to the U₄ ^(↓) population via the unlearning, top-down unlearningpathway 131. Anti-Hebb learning based on the firing activity of theneurons in the neuron populations L₃ ^(↑) and U₂ ^(↓) is applied toadjust the synaptic weights of the weighted synaptic connections in theneural network 140.

FIG. 19 illustrates a neural network structure 140 trained insupervised, unsupervised and reinforcement learning with learning rulesapplied, in accordance with an embodiment of the invention. Anti-Hebblearning based on the firing activity of the neurons in the neuronpopulations U₃ ^(↑) and L₂ ^(↓) is applied to adjust the synapticweights of the weighted synaptic connections in the neural network 140.

After training, the neural network 140 will approximate the feedforwardstructure 100 in FIG. 8.

FIG. 20 illustrates a flowchart of an example process 180 for training aneural network in supervised, unsupervised and reinforcement learning byimplementing an embodiment of spiking computation, in accordance with anembodiment of the invention. In one embodiment, the neural network maycomprises a hierarchical routing fabric for neural modules, wherein therouting fabric provides two-way information flow and structuralplasticity. Routing of information between the neural modules isadaptive. In one example, each neural module includes a plurality ofincoming connections such that each incoming connection has apredetermined address, and each neural module includes a plurality ofoutgoing connections such that each outgoing connection targets anincoming connection in a neural module among the multiple neuralmodules. The routing fabric selectively routes signals among the neuralmodules based on a reconfigurable hierarchical organization. The routingfabric may comprise a routing table for each neural module, wherein eachrouting table includes routing information for a router to direct anincoming signal to another neural module. In one example, the routingtable is within a neural module. In another example, the routing tablemay be external to the neural module.

In process block 181, the neural modules are established wherein eachneural module comprises multiple neurons. In process block 182, aweighted synaptic connection between every neuron in a first neuralmodule and a corresponding neuron in a second neural module isestablished. In process block 183, for every time step, the operationalstate of every neuron in every neural module is updated, learning rulesare applied to the weighted synaptic connections for learning falsenegatives or unlearning false positives, and neuronal firing events arecommunicated. After process block 183, the invention repeats processblock 183.

FIG. 21 is a high level block diagram showing an information processingcircuit 300 useful for implementing one embodiment of the presentinvention. The computer system includes one or more processors, such asprocessor 302. The processor 302 is connected to a communicationinfrastructure 304 (e.g., a communications bus, cross-over bar, ornetwork).

The computer system can include a display interface 306 that forwardsgraphics, text, and other data from the communication infrastructure 304(or from a frame buffer not shown) for display on a display unit 308.The computer system also includes a main memory 310, preferably randomaccess memory (RAM), and may also include a secondary memory 312. Thesecondary memory 312 may include, for example, a hard disk drive 314and/or a removable storage drive 316, representing, for example, afloppy disk drive, a magnetic tape drive, or an optical disk drive. Theremovable storage drive 316 reads from and/or writes to a removablestorage unit 318 in a manner well known to those having ordinary skillin the art. Removable storage unit 318 represents, for example, a floppydisk, a compact disc, a magnetic tape, or an optical disk, etc. which isread by and written to by removable storage drive 316. As will beappreciated, the removable storage unit 318 includes a computer readablemedium having stored therein computer software and/or data.

In alternative embodiments, the secondary memory 312 may include othersimilar means for allowing computer programs or other instructions to beloaded into the computer system. Such means may include, for example, aremovable storage unit 320 and an interface 322. Examples of such meansmay include a program package and package interface (such as that foundin video game devices), a removable memory chip (such as an EPROM, orPROM) and associated socket, and other removable storage units 320 andinterfaces 322 which allow software and data to be transferred from theremovable storage unit 320 to the computer system.

The computer system may also include a communication interface 324.Communication interface 324 allows software and data to be transferredbetween the computer system and external devices. Examples ofcommunication interface 324 may include a modem, a network interface(such as an Ethernet card), a communication port, or a PCMCIA slot andcard, etc. Software and data transferred via communication interface 324are in the form of signals which may be, for example, electronic,electromagnetic, optical, or other signals capable of being received bycommunication interface 324. These signals are provided to communicationinterface 324 via a communication path (i.e., channel) 326. Thiscommunication path 326 carries signals and may be implemented using wireor cable, fiber optics, a phone line, a cellular phone link, an RF link,and/or other communication channels.

In this document, the terms “computer program medium,” “computer usablemedium,” and “computer readable medium” are used to generally refer tomedia such as main memory 310 and secondary memory 312, removablestorage drive 316, and a hard disk installed in hard disk drive 314.

Computer programs (also called computer control logic) are stored inmain memory 310 and/or secondary memory 312. Computer programs may alsobe received via communication interface 324. Such computer programs,when run, enable the computer system to perform the features of thepresent invention as discussed herein. In particular, the computerprograms, when run, enable the processor 302 to perform the features ofthe computer system. Accordingly, such computer programs representcontrollers of the computer system.

From the above description, it can be seen that the present inventionprovides a system, computer program product, and method for implementingthe embodiments of the invention. References in the claims to an elementin the singular is not intended to mean “one and only” unless explicitlyso stated, but rather “one or more.” All structural and functionalequivalents to the elements of the above-described exemplary embodimentthat are currently known or later come to be known to those of ordinaryskill in the art are intended to be encompassed by the present claims.No claim element herein is to be construed under the provisions of 35U.S.C. section 112, sixth paragraph, unless the element is expresslyrecited using the phrase “means for” or “step for.”

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method for feature extraction, comprising: at aneural network comprising a plurality of neural modules interconnectedvia a plurality of weighted synaptic connections: receiving a firstinput; extracting one or more input features from the first input as thefirst input propagates through the neural network via at least one ofthe weighted synaptic connections; training the neural network toassociate the one or more input features with the first input byapplying a learning rule to at least one of the weighted synapticconnections to strengthen conductance of the at least one weightedsynaptic connection; and after the training, in response to receiving asecond input comprising first input with noise, reproducing the firstinput without noise via the trained neural network; wherein the trainingcomprises unlearning network-created information including falsepositives by copying firing events generated by a neural modulepositioned along a learning, top-down pathway of the neural network, andproviding the copied firing events as input to another neural modulepositioned along an unlearning, bottom-up pathway of the neural network;and wherein each neural module comprises an electronic circuit includingmultiple neurons.
 2. The method of claim 1, wherein: each neuron in eachneural module is connected to a corresponding neuron in another neuralmodule via a weighted synaptic connection of the weighted synapticconnections.
 3. The method of claim 2, further comprising: at the neuralnetwork: adaptively updating weights of the weighted synapticconnections as a function of firing events of interconnected neurons. 4.The method of claim 1, wherein the training the neural network comprisesapplying at least one learning rule to the at least one weightedsynaptic connection to adjust conductance of the at least one weightedsynaptic connection as the first input propagates through the neuralnetwork via the at least one weighted synaptic connection.
 5. A systemcomprising a computer processor, a computer-readable hardware storagemedium, and program code embodied with the computer-readable hardwarestorage medium for execution by the computer processor to implement amethod comprising: at a neural network comprising a plurality of neuralmodules interconnected via a plurality of weighted synaptic connections:receiving a first input; extracting one or more input features from thefirst input as the first input propagates through the neural network viaat least one of the weighted synaptic connections; training the neuralnetwork to associate the one or more input features with the first inputby applying a learning rule to at least one of the weighted synapticconnections to strengthen conductance of the at least one weightedsynaptic connection; and after the training, in response to receiving asecond input comprising the first input with noise, reproducing thefirst input without noise via the trained neural network; wherein thetraining comprises unlearning network-created information includingfalse positives by copying firing events generated by a neural modulepositioned along a learning, top-down pathway of the neural network, andproviding the copied firing events as input to another neural modulepositioned along an unlearning, bottom-up pathway of the neural network;and wherein each neural module comprises an electronic circuit includingmultiple neurons.
 6. The system of claim 5, wherein: each neuron in eachneural module is connected to a corresponding neuron in another neuralmodule via a weighted synaptic connection.
 7. The system of claim 6, themethod further comprising: at the neural network: adaptively updatingweights of the weighted synaptic connections as a function of firingevents of interconnected neurons.
 8. The system of claim 5, wherein thetraining the neural network comprises applying at least one learningrule to the at least one weighted synaptic connection to adjustconductance of the at least one weighted synaptic connection as thefirst input propagates through the neural network via the at least oneweighted synaptic connection.
 9. A computer program product comprising acomputer-readable hardware storage device having program code embodiedtherewith, the program code being executable by a computer to implementa method comprising: at a neural network comprising a plurality ofneural modules interconnected via a plurality of weighted synapticconnections: receiving a first input; extracting one or more inputfeatures from the first input as the first input propagates through theneural network via at least one of the weighted synaptic connections;training the neural network to associate the one or more input featureswith the first input by applying a learning rule to at least one of theweighted synaptic connections to strengthen conductance of the at leastone weighted synaptic connection; and after the training, in response toreceiving a second input comprising the first input with noise,reproducing the first input without noise via the trained neuralnetwork; wherein the training comprises unlearning network-createdinformation including false positives by copying firing events generatedby a neural module positioned along a learning, top-down pathway of theneural network, and providing the copied firing events as input toanother neural module positioned along an unlearning, bottom-up pathwayof the neural network; and wherein each neural module comprises anelectronic circuit including multiple neurons.
 10. The computer programproduct of claim 9, wherein: each neuron in each neural module isconnected to a corresponding neuron in another neural module via aweighted synaptic connection of the weighted synaptic connections. 11.The computer program product of claim 10, the method further comprising:at the neural network: adaptively updating weights of the weightedsynaptic connections as a function of firing events of interconnectedneurons.
 12. The computer program product of claim 9, wherein thetraining the neural network comprises applying at least one learningrule to the at least one weighted synaptic connection to adjustconductance of the at least one weighted synaptic connection as thefirst input propagates through the neural network via the at least oneweighted synaptic connection.